Bag bottom



Nov. 18, 1969 s. J. PERLMAN 3,478,952

BAG BOTTOM Filed April 14, 1967 INVENTOR United States Patent O3,478,952 BAG BOTTOM Sol J. Perlman, Baltimore, Md., assignor to Vac PacManufacturing Company, Inc., Baltimore, Md., a corporation of MarylandFiled Apr. 14, 1967, Ser. No. 630,961

Int. Cl. B65d 33/22 U.S. Cl. 22957 3 Claims ABSTRACT OF THE DISCLOSUREAn improved method of adhering the bottom of heat shrinkablepolyalkylene terephthalate film bags, such as those used to shrinkpackage frozen turkeys, is disclosed. The bags are adhered at the bottomwith two distinct adhesive bands. The inner band is a thermoplasticadhesive which allows a measure of peelability and shock absorbability,whereas the outer adhesive band is a high strength thermosettingadhesive. The contained product may even be cooked within the bag.

BACKGROUND OF THE INVENTION Polyalkylene terephthalate films have beenknown to the prior art, and have previously been used for packagingapplications. When the term polyalkylene terephthalate is used in thepresent specification, it is to be understood to apply to polymericlinear terephthalate esters formed by reacting a glycol of the series HO(CH OH wherein n is an integer of 2 to 10, inclusive, with'terephthalicacid or a lower alkyl ester of terephthalic acid, wherein the alkylgroup contains 1-4 carbon atoms, such as, for example, dimethylterephthalate. The preparation of polyalkylene terephthalates isdisclosed in U.S. Patent No. 2,465,319 to Whinfield and Dickson, thedisclosure of which is hereby incorporated by reference. The most widelyused and commercially attractive polyalkylene terephthalate material ispolyethylene terephthalate, such as the product sold under thecommercial trade name Mylar. Polyethylene terephthalate is generallyproduced by an ester interchange between ethylene glycol and dimethylterephthalate to form bis-2-hydroxy ethyl terephthalate monomer, whichis polymerized under reduced pressure and elevated temperature topolyethylene terephthalate. The alkylene terephthalate polyesters may bereadily formed into films, for example, by melt casting or by tubularblown film extrusion.

The polyalkylene terephthalate films which are used in the shrinkpackaging field are films which have been oriented and heat set atelevated temperatures and restrained shrinkage. Polyethyleneterephthalate film in substantially amorphou form may be oriented, forexample, by stretching the film substantially equally in both lineardirections, generally about 2- to 3 A-fold. Generally, heat setting ofthe film is accomplished after the orientation and is accomplished whilemaintaining the film under tension at a temperature above 100 C.

The heat shrinkable polyester film produced from polyalkyleneterephthalates is not heat sealable, in the sense that that term is usedin the packaging field. The adhesive force developed between contactinglayers of the polyester film during heat sealing treatments is not ofsufficient strength for shrink packaging applications. However, itshould be realized that the heat shrinkable polyalkylene terephthalatefilms might be classified as heat sealable for other applicationswherein difierent requirements prevail.

The prior art has produced polyalkylene terephthalate 3,478,952 PatentedNov. 18, 1969 ice film ,bags for packaging applications bysimultaneously applying the bottom seal adhesive and the longitudinal(backor side) seal adhesive to a flat sheet of the plastic film. Thefilm sheet is then folded and thus adhered into the desired bag shape.Prior art polyalkylene terephthalate adhesives are disclosed by, forexample, U.S. Patents 2,820,735, 2,961,365 and 3,022,192, thedisclosures of which are hereby incorporated by reference. It is knownto use a thermoplastic adhesive alone on the bottom of the bag and athermosetting adhesive alone to form the longitudinal back seal of thebag. Due to the use of the thermoplastic adhesive alone on the bottom ofthe bag, the packager must reverse, e.g., turn inside out, the bag inorder to obtain adequate bottom seam strength, which is an extra step.The use of thermosetting adhesives alone on the bottom seal of such bagsproved commercially unsatisfactory for two major reasons. First, the bagwas subjected to shock while loading the product, causing the "bag tofail, because of poor shock absorbing characteristics. In packaging, theproduct is generally rammed into a bag and such bags had insufiicientbottom edge shock absorbability and/or peelability. By peelability ismeant the ability of the adhesive to allow at least a portion of thepolyalkylene terephthalate films at the adhered band to separate fromone another without film tear. Second, after the product is placed inthe polyester bag and the bag is closed at the top, the film is heatshrunk upon the contained article. The shrinking cycle develops quitehigh stresses in polyalkylene terephthalate films and has caused theprior art bags to tear when the adhesive had insufiicient peelability.

The polyalkylene terephthalate film is a biaxially oriented filmexhibiting very high shrink energies of at least 700 p.s.i., and usually1000 p.s.i. or above, preferably about 1700 p.s.i., or even higher. Suchfilms have been previously used in shrink packaging. The properties ofshrink packaging polymeric films is discussed by Robert D. Lowry in TheGrowth of Shrink Packaging, Modern Packaging Encyclopedia, pages 288 to296, 1963 the disclosure of which is hereby incorporated by reference.

The polyalkylene terephthalate film bags produced by known methods havenot been able to be used for cooking a contained food product, as thethermosetting adhesives did not have the requisite degree of peelabilityto withstand the resultant high film stresses, and the thermoplasticadhesives lost too much strength at the cooking temperatures. Ingeneral, an extended heating cycle at temperatures of 210 F. or abovehave caused failure in the known polyalkylene terephthalate film bags.

BRIEF DESCRIPTION OF THE INVENTION The above disadvantages of the priorart polyalkylene terephthalate film bags in shrink packagingapplications are overcome by the use of two adhesive bands at the bottomedge of the polyester film bag. The outer band, e.g., the band closestto the outer edge of the bag, is a thermosetting adhesive. The innerband, which may be adjacent to the outer band or may be spacedtherefrom, is a thermoplastic adhesive. The outer thermosetting adhesiveimparts the required strength and heat resistance to the adhered sectionor band, while the inner, thermoplastic adhesive imparts the desiredpeelability," which allows the bag to absorb shock.

DESCRIPTION OF THE DRAWINGS The invention will be more readilyunderstood with reference to the accompanying set of drawings, whereinFIGURE 1 represents a side view of the polyalkylene terephthalate bag ofthe invention and FIGURE 2 represents across-section of the bag ofFIGURE 1, taken along the line 2-2. In FIGURE 1, a polyalkyleneterephthalate bag 1 is shown which comprises a top 3 and a bottom 2. Thebag is illustrated as formed from a tubular portion of polyalkyleneterephthalate film, although the present commercial bags are formed froma flat piece of film by making a suitable longitudinal seam, as known ofthe art. The bottom of the bag is sealed by an outer adhesive layer 4and an inner adhesive layer 5. The adhesive bands are shown as beingseparated from one another, but they may, if desired, be adjacent toeach other.

In FGURE 2, the bag 1 is shown in cross-section. Polyalkyleneterephthalate film tube 6 is closed at one end (the bottom end) by meansof an inner adhesive band and an outer adhesive band 4 (not shown).

GENERAL DESCRIPTION OF THE INVENTION The heat shrinkable polyalkyleneterephthalate film bags of the present invention exhibit greatly reducedtendencies to tear and otherwise be adversely affected by mechanicalshock and stresses caused by loading the product into a bag, by heatshrinking the bag around the contained product, or by subjecting the bagto extended heat treatment, such as, for example, subjecting thepackaged product to immersion in boiling water to cook the containedproduct.

The key to the successful performance of the bags produced by thepresent invention resides in the peelability of the inner adhesive bandand the high heat resistance and high cohesive strength exhibited by theouter adhesive band.

The inner thermoplastic adhesive band functions to absorb shocksimparted to the adhesive band and the outer thermosetting adhesive bandfunctions to provide an adhesive area of high cohesive strength and highheat resistance. These bands may be of any desired width, and may beadjacent one another or spaced apart from each other. In the case ofpolyalkylene terephthalate film bags suitable for the packaging offrozen turkeys, it is preferred to use a wide laydown of each adhesivelayer and to have no space between the layers, e.g., the layers areadjacent. Generally, each adhesive layer or band will be at least A:"wide.

Polyalkylene terephthalate films are known to have high tensile strengthbut their tear resistance when subjected to shock, especially in themachine direction, is low. Therefore, the adhesive utilized must becapable of exhibiting a shock-absorbing ability during the heatshrinkage cycle, for otherwise the film will tear due to the highstresses involved.

The adhesive bands may be applied by any of the methods known to theart. For example, they may be applied by brushes or a flexograp'hicpress. Four rolls may be used to transfer the adhesive to a desired bagpattern. For instance, for round bottomed bags it has generally beendesired to apply the adhesive in a sine curve laydown, whereas astraight laydown is generally used for flat bottomed bags. Other bagshapes and adhesive laydown patterns may be used in the practice of thepresent invention. It will readily be appreciated that the bags producedaccording to the present inven tion need not be reversed beforepackaging, as normally done, thereby allowing the bag to be printed onits outer surface either before or after the application of the adhesivelayers.

After the bags are manufactured, the product may be loaded through theopen end, and the bag then air evacuated and closed, for instance, bymeans of a metal or plastic clip, as known to the art, and then the bagmay be shrunk upon the contained product, for instance byimmersion inhot water baths, e.g., at 210 F. As mentioned, it is possible to cookfood for any necessary length of time while the food is contained in thepresent bag. Polyethylene terephthalate films generally melt at atemperature of about 425 to 450 F. and generally the cookingtemperatures should not be higher than 4 350 F. The ability to cookcontained food products in polyalkylene terephthalate bags is unique, asfar as known, to the bags produced by the present invention.

A wide variety of adhesives may be used in the practice of the presentinvention. The inner, thermoplastic adhesive may be any thermoplasticadhesive which exhibits adhesive characteristics to polyalkyleneterephthalate films. Particularly preferred thermoplastic polyesteradhesives, for example, are described in the Chapman U.S. Patent3,142,434, the disclosure of which is hereby incorporated by reference.Among other thermoplastic adhesives may be mentioned, by way of example,and not of limitation, the polyamide adhesives disclosed'in US. Patent2,379,413, especially those of molecular weights of 6,000 to 9,000 andproduced by the reaction of dimerized linoleic acid and ethylenediamine. Other thermoplastic adhesives for polyalkylene terephthalateare known to the art and may be utilized.

The outer thermosetting adhesive is generally a two component, or twocan, system. Polyurethane and polyester adhesive are particularlypreferred but any of the thermosetting adhesive for polyalkyleneterephthalate films known to the art may be utilized. In general, theamount of catalyst when utilized in such a system must be closelywatched in order to avoid gellation in the equipment involved. It isgenerally preferred to cure'the thermosetting resin at a quite low rate.If desired, a modified isocyanate curing agent, may be added topolyester adhesives.

As a polyester adhesive there may be used, for instance, astyrenemodified polyethylene glycol polypropylene glycoladipate-maleate, as is known to the art. The selection of a propercatalyst for any given thermosetting polyalkylene terephthalate filmadhesive is within the skill of those in the art.

The invention will be understood more readily by reference to thefollowing examples; however, these examples are intended to illustratethe invention and are not to be construed to limit the scope of theinvention.

Example 1 Two adhesive bands were applied by way of a fiexographic pressto oriented heat shrinkable polyethylene terephthalate film 0.0005 inchthick, in a contour to give a round-bottom bag with a longitudinalcenter seal in one side. The inner band was of an inch thick and was thepolyester adhesive sold under the identification 469 68 polyester by DuPont (produced by the process of U.S. Patent 3,142,434). The outeradhesive was a thermosetting adhesive, based on a polyurethane resin.This was a two part adhesive system under the designation 712- 602 and713-806. The 7l2602 material is a polyol composition exhibiting aBrookfield viscosity at 77 F. of 400-600 cps. at 25% solids in methylethyl ketone. The 713-806, which was used in an amount corresponding to5% by weight of the 712-602, was a polyurethane prepolymer which was a75% solid solution in methyl ethyl ketone, exhibiting a viscosity ofthat of a medium syrup. After 5 days aging at room temperature to insurecure of the adhesive band, the bag, which was of 16 inch lay fiat width,was loaded with a turkey, vacuum evacuated and the open end was sealedby a metal clip. The bag was immersed in a hot water bath (at F.) for 5minutes thereby causing the film to shrink around the turkey. Thpackaged turkey was then frozen.

Example 11 The frozen package turkey produced according to Example I wascooked in a pressure cooker at 250 F. for three hours, until done.Examination of the polyethylene terephthalate film showed no rips ortears in the bag and a tight adhesive band.

Example III Example I was repeated except different adhesives were used.The inner, thermoplastic adhesive was prepared as follows: 30 parts of acopolyester having a relative viscosity of 1.6 prepared by reactingethylene glycol with 33 mol percent dimethyl terephthalate, 17 molpercent dimethyl isophthalate and 50 mol percent dimethyl sebacate underester-interchange conditions followed by polymerizing, were stirredtogether with 4.5 parts of triethyl citrate, 1.5 parts of Syloid 162silica micron silica, Davidson Chem-W. R. Grace Co.) and 0.45 part oflauryl alcohol into a volatile organic solvent mixture consisting of32.9 parts of methyl ethyl ketone, 3.5 parts of methyl isobutyl ketoneand 21 parts of xylene.

The outer thermosetting adhesive was a two can polyurethane adhesive.One component consisted of a urethane prepolymer of toluene diisocyanateand a mixture of trimethylol propane and diethylene glycol adipate, themixture having hydroxyl functionality of 2.5. The prepolymer contained28% free NCO. The second component of the two can polyurethane adhesivewas a polyol composition containing the reaction product ofpentaerythritol with 5 mols of propylene oxide, together withtriethylene diamine catalyst. The two components of the polyurethaneadhesive were used in a proportion to give a final NCOzOH ratio of 1.05:1.0. The triethylene diamine was present in an amount equal to 0.5% byweight of the total adhesive composition.

The packaged turkey was examined after freezing and after cooking andthe polyethylene terephthalate film exhibited no signs of rips or tears,and the outer adhesive band remained intact.

Example IV Example III was repeated but in place of the polyurethanethermosetting adhesive was used a polyester thermosetting adhesive whichwas a styrene-modified polyethylene glycol-polypropylene glycoladipate-maleate. The final product produced by this example was similarto the product of Example HI.

In a preferred form of the invention, the polyalkylene terephthalatefilm bag is heat sealed at the adhesive band to reduce the peelabilityof the thermoplastic adhesive. Such heat treated, or heat sealed,adhesive bands allow the production of bags which can withstand greatershocks.

The polyalkylene terephthalate film bags of the present invention arepreferably used to package food products, most preferably turkeys, butit is to be understood that these bags may be used to package a widevariety of other items as well. When food items are packaged within thebags of the present invention, the food may be cooked in the bag itself,for example, by immersion in boiling water. In general, the cookingcycle will be at a temperature of about 210 F. to 350 F. and for atleast ten minutes and until the product is cooked.

I claim:

1. A polyalkylene terephthalate film bag suitable for shrink packaging,said bag comprising oriented heat shrinkable polyalkylene terephthalatefilm in the shape of an open-end bag, at least the bottom of said bagbeing formed by adhering the polyalkylene terephthalate film to itself,the adhesive used to adhere said polyalkylene terephthalate film beingthe combination of (1) an inner band of thermoplastic polyalkyleneterephthalate film adhesive and (2) an outer band of thermosettingpolyalkylene terephthalate film adhesive.

2. The product claimed in claim 1 wherein said polyalkyleneterephthalate film bag is polyethylene terephthalate film bag, and eachadhesive band is at least A5" wide.

3. The product as claimed in claim 2 wherein said thermoplastic adhesiveis a thermoplastic polyester or polyamide polyethylene terephthalatefilm adhesive, and said thermosetting adhesive is a thermosettingpolyester or polyurethane polyethylene terephthalate film adhesive.

References Cited UNITED STATES PATENTS 2,880,925 4/ 1959 Klassing et a1.22953 3,062,373 11/1962 Reynolds 206- 3,204,760 9/ 1965 Whiteford 22948DAVID M. BOCKENEK, Primary Examiner U.S. Cl. X.R. 229-48, 33

